Method of making electric heating elements

ABSTRACT

An electric heating element, formed by winding a resistor wire about a refractory core, disposing terminal portions of said wound wire within respective ones of a pair of openings extending longitudinally through the core, inserting flexible terminal conductors into respective core openings to electrically engage with respective terminal portions, inserting the core assembly within a tubular metal sheath with the terminal conductors extending outwardly of the sheath to form flexible leads for connection to a source of electrical energy, filling the sheath with granular refractory material and submitting the sheath to a side pressing force to compact the refractory material.

United States Patent 1 Drugmand 1 May 28, 1974 METHOD OF MAKING ELECTRIC HEATING ELEMENTS [75] Inventor: Lester D. Drugmand, Pittsburgh, Pa.

[73] Assignee: Emerson Electric Co., St. Louis, Mo.

[22] Filed: Feb. 6, 1970 211 App]. No.: 9,158

[52] US. Cl 29/611, 29/615, 29/619, 219/544, 338/238, 338/240 [51] Int. Cl H0lc 17/00 [58] Field of Search 338/238, 240, 273-276, 338/243, 250; 29/611, 613-615, 619, 621;

[56] References Cited UNITED STATES PATENTS 2,718,577 9/1955 Sherk 338/240 X 3,071,748 l/1963 Lucia..... 3,080,543 3/1963 Boggs 338/238 x FOREIGN PATENTS OR APPLICATIONS 256,908 8/1926 Great Britain 338/240 Primary Examiner-Darrcll L. Clay Attorney, Agent, or Firm-Michael Williams [57] ABSTRACT An electric heating element, formed by winding a resistor wire about a refractory core, disposing terminal portions of said wound wire within respective ones of a pair of openings extending longitudinally through the core, inserting flexible terminal conductors into respective core openings to electrically engage with respective terminal portions, inserting the core assembly within a tubular metal sheath with the terminal conductors extending outwardly of the sheath to form flexible leads for connection to a source of electrical energy, filling the sheath with granular refractory material and submitting the sheath to a side pressing force to compact the refractory material.

1 Claim, 9 Drawing Figures :mmnmm I 3312.580 SuEET 2 OF 2 INVENTOR. LESTER D. DRUGMAND METHOD OF MAKING ELECTRIC HEATING ELEMENTS BACKGROUND AND SUMMARY In the prior art know to applicant, solid nickel wires extend the full length of a pair of openings in a ceramic core to make pressure contact with the respective ends of a resistance wire which is wound about the core and has its terminal portions within the core openings. The solid wires extend outwardly of a sheath enclosing the core and provide leads for connection to a source of electrical energy. The solid wires make it difficult to bend the leads any appreciable amount close to the sheath end and thus the heater produced in accordance with the prior art is unsuitable for certain applications which require practically a right angle bend in the leads at the sheath end.

In addition, the stiffness of the solid leadwires concentrates stresses in the area where a stranded leadwire is connected to the solid leadwire by means of a cylindrical type connector, and under some conditions of vibration and flexing this very often causes the leadwire to break at that point.

I have discovered that the disadvantages of the prior art constructions may be overcome by inserting flexible terminal conductors into the core openings and continuing such conductors outwardly of the sheath to form flexible leadwires which may be bent at rightangles at sheath end, if the application so requires. Therefore, the principal object of my invention is to provide new and improved electric heaters and methods of making the same.

DESCRIPTION OF THE DRAWINGS In the drawings accompanying this description and forming a part of this specification, there is shown, for purpose of illustration, an embodiment which my invention may assume, and in these drawings:

FIG. 1 is a broken sectional view through a core, showing a resistor conductor wound thereon, v

FIG. 2 is a broken elevational view of a flexible lead wire used in the embodiment of my invention,

FIG. 3 is a perspective view of a contact strip shown in FIG. 1,

FIG. 4 is a transverse section corresponding to the line 4-4 of FIG. 1,

FIG. 5 is a broken sectional view of an assembly of the parts shown in FIGS. 1, 2 and 3,

FIG. 6 is a broken sectional view illustrating a further step in the assembly,

FIG. 7 is a transverse section corresponding to the line 7-7 of FIG. 6,

FIG. 8 is a broken sectional view illustrating a further step in the assembly, and

FIG. 9 is a broken sectional viewof the completed heater.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1, 3 and 4, a resistance conductor, such as Nichrome wire 10, is helically wound about a cylindrical core 11. The core may be an extrusion and is preferably formed of crushable refractory material. The core has a pair of openings 12 and 13 extending longitudinally therethrough and opposite terminal portions of the wound wire are disposed in such openings.

The ends of the wires may be disposed in the core openings, or as shown in the drawings, such ends are connected to contact strips 14. Each contact strip is formed of a metal which will withstand the internal heat of the heating element, and a nickel or nickel alloy has been found suitable.

As seen in FIGS. 1 and 3, each contact strip is formed with an elongated rectilinear portion 15 which is slightly crowned in cross-section, and an end portion 16 which fits about an adjoining end portion of the core. The end portion 16 terminates in a clip 17 which is crimped over the wire end. One contact strip 14 is disposed in the core opening 12 and is accessible from the end 11a of the core and the other contact strip 14 is disposed in the core opening 13 and is accessible from the end 11b of the core.

Flexible leads, each like the lead 18 shown in FIG. 2, are disposed within the core openings 12 and 13 and electrically connected to respective contact strips 14. The leads 18 are here shown as bare stranded wire, preferably of nickel or nickel alloy. Since the stranded wires have a tendency to fray at the lead end, it is preferable to tin an end of the lead, as shown at 19. A lead 18 is inserted into core opening 12, with its tinned end foremost, from the core end 11a, and the other lead is inserted into core opening 13, with its tinned end foremost, from the core end 1117, so as to insure that the contact strips 14 are not displaced from the respective core openings. The leads are inserted into respective core openings 21 sufficient amount so that the tinned ends project beyond the openings and thereafter such tinned ends are trimmed away. The cross-section of each lead 18 is related to the transverse size of the core opening so that it closely fits therein and thereby tightly presses against the contact strip 14 for good electrical connection. The stranded'wire leads may be twisted about their longitudinal axis to provide stiffness for insertion of the lead in the core opening. FIG. 5 shows the leads 18 in place, with both leads extending the length of the core and having portions extending from the core end 11a. The core is formed with slots 20 at each end so that the clip portion of each contact strip does not materially extend beyond the periphery of the core.

With proper support, the core assembly shown in FIG. 5 may be used as an electric heater by connecting the leads 18-18 to a source of electrical energy. However, it is preferable to insert the core assembly of FIG. 5 longitudinally within a metal alloy tube 21, as seen in FIG. 6. A crushable refractory busing 121 is inserted within an end of the tube or sheath 21 and abutted against the core end and a further bushing 22 formed of a combustible material, such as polyethylene, is inserted into the tube end and abutted against the bushing 121, and the tube end is then turned inwardly, as shown at 23, to lock the bushings in place. Bushings 21, 22 are formed with openings to pass the lead wires 18 which are long enough to extend some distance outwardly of the tube end.

A centering bushing 24 is inserted into the opposite end of the tube 21 in close fitting relation with the inner wall of the tube, and this bushing has an annular shoulder 25 which closely fits about the core periphery. The bushing is also formed of crushable refractory material, and is formed with slots 26 in its periphery. The assembly thus far formed is uprighted, with tube end 21a uppermost, and powdered refractory material 27 is fed into such tube end, the material passing through the bushing slots 26 and the tube 21 is preferably subjected to a vibrating action to insure that the material fills all voids within the tube.

The tube filling is continued until the refractory material fills the end 21a of the tube. Thereafter, sufficient material is removed to make space for a metal disc 28 (see FIG. 8) which is inserted into the tube end 21a with a force fit. The disc 28 is hermetically sealed within the tube, as by the heli-arc weld 29.

Thereafter, the tube 21 is subjected to a side pressing force, such as by a swaging operation, to reduce its cross-sectional size. Such swaging also crushes the core 11, and bushings 121 and 24 and they form a rock-like homogeneous mass with the refractory material, as suggested in FIG. 9. Heaters have been made in accordance with the foregoing description, wherein the outside diameter of the tube 21 was about .415 inches before swaging, and such-outside diameter was reduced to about .373 inches after swaging. The swaging operation results in astraightening of the inwardly turned portion 23 of the tube end, as seen in FIG. 9.

After the swaging operation, the polyethylene bushing 22 is burned out in any suitable manner and flexible insulating sleeves 30 disposed over the extending ends of the flexible leads 18. Thereafter, a cement, such as a suitable air-setting, cement 31, is inserted into the space in thetube end which is formed when the polyethylene bushing 22 is burned out, such cement, when set, firmly locking the insulating sleeves 30 in place. In view of the fact that the leads 18 are flexible, such leads may be sharply bent immediately exteriorly of the cement 31. The swaging operation also firmly presses the contact strips 14 and leads '18 into good electrical engagement, and since the leads are stranded, they conform to the configuration of the strips to further improve electrical interengagement.

I claim: 7

1. The method of forming an electrical heater, comprising winding a resistance conductor helically about a core, disposing bare terminal portions of said resistance conductor within respective ones of a pair of longitudinal openings in said core, and inserting bare, multistrand flexible terminal conductors into respective 4 ones of said longitudinal openings, each of said openings being of a transverse size proportional to the combined transverse size of a bare terminal portion and a terminal conductor so that the inserting operation of said terminal conductors abuts the same with respective terminal portions for electrical interconnection, said terminalconductors being inserted into said core openings with end portions extending from said core to form integral flexible leads for connection to a source of energy, said leads being capable of being bent sharply at an angle to the axis of said core openings adjacent to their extension from said core,

the foregoing steps producing a core assembly, and said method further including the steps of inserting the core assembly thus produced longitudinally within a tubular metal sheath having an internal transverse size greater than the external transverse size of said core assembly, inserting a refractory bushing into oneend of said sheath, such bushing having a pair of openings to pass said terminal conductors, inserting a polyethylene bushing into said one end of said sheath against said refractory bushing, 'said polyethylene bushing having a pair of openings to pass said terminal conductors, turning jin said one sheath end to lock said polyethylene and refractory bushings'in place, applying a refrac tory apertured centering bushing over said core at the other end of said sheath to space the core from the inner wall of said sheath, inserting powdered refractory'material through the apertures in said centering bushing to fill all voids in said sheath, inserting a closure disc into said other sheath end with a force fit, hermetically sealing said closure disc to saidsheath by means of a fusion joint, subjecting said sheath to a side'pressing force to com-' pact said powdered refractory material, burning out said polyethylene bushing, inserting flexible insulating sleeves over those portions of said terminal conductors which extend through the void left when said polyethylene bushing has been burned out and outwardly of said sheath, and filling said void with an air setting cement to close said other sheath end and lock said insulating sleeves in position. i 

1. The method of forming an electrical heater, comprising winding a resistance conductor helically about a core, disposing bare terminal portions of said resistance conductor within respective ones of a pair of longitudinal openings in said core, and inserting bare, multistrand flexible terminal conductors into respective ones of said longitudinal openings, each of said openings being of a transverse size proportional to the combined transverse size of a bare terminal portion and a terminal conductor so that the inserting operation of said terminal conductors abuts the same with respective terminal portions for electrical interconnection, said terminal conductors being inserted into said core openings with end portions extending from said core to form integral flexible leads for connection to a source of energy, said leads being capable of being bent sharply at an angle to the axis of said core openings adjacent to their extension from said core, the foregoing steps producing a core assembly, and said method further including the steps of inserting the core assembly thus produced longitudinally within a tubular metal sheath having an internal transverse size greater than the external transverse size of said core assembly, inserting a refractory bushing into one end of said sheath, such bushing having a pair of openings to pass said terminal conductors, inserting a polyethylene bushing into said one end of said sheath against said refractory bushing, said polyethylene bushing having a pair of openings to pass said terminal conductors, turning in said one sheath end to lock said polyethylene and refractory bushings in place, applying a refractory apertured centering bushing over said core at the other end of said sheath to space the core from the inner wall of said sheath, inserting powdered refractory material through the apertures in said centering bushing to fill all voids in said sheath, inserting a closure disc into said other sheath end with a force fit, hermetically sealing said closure disc to said sheath by means of a fusion joint, subjecting said sheath to a side pressing force to compact said powdered refractory material, burning out said polyethylene bushing, inserting flexible insulating sleeves over those portions of said terminal conductors which extenD through the void left when said polyethylene bushing has been burned out and outwardly of said sheath, and filling said void with an air setting cement to close said other sheath end and lock said insulating sleeves in position. 